In the design of electric storage batteries, it is frequently desirable to include a device for monitoring and indicating the state-of-charge, and hence the relative strength of the battery. Rudimentary devices for providing such an indication were developed very early in the art, as described, for example, in U.S. Pat. Nos. 964,995 to P. M. Marko and 1,010,337 to H. H. Kemph. The device described in each of these patents includes a volt meter which is secured to the cover of the battery, and which is connected to the terminals of the battery by wire leads. The volt meter disclosed in these patents does not continually monitor the condition of the battery. Instead, the meters are associated with simple push button switches which, when depressed, serve to electrically connect the meters to the terminals. In this way, the strength of each battery can be ascertained by depressing the switch, causing a short circuit through the volt meter. The meter then indicates the voltage of the battery.
More recent devices for indicating the state-of-charge of an electric storage battery are described in U.S. Pat. Nos. 4,248,942 to R. L. Eby et al, 4,467,017 to T. R. Jackson and 4,665,370 to Holland. The first of these patents discloses an electric monitoring and indicating device which includes a light emitting diode (LED) which indicates when the battery has reached a preselected state-of-charge. The device continues to provide such an indication until the battery has been discharged to a second, preselected state.
The Jackson '017 patent contemplates a completely different state-of-charge monitoring device, and comprises a secondary electrolytic cell located within the electrolyte of a larger, primary cell. The secondary cell is intended to provide an output which is dependent upon the characteristics of the electrolyte, and which is said to vary proportionally with the state-of-charge of the primary cell. The structure of the device disclosed in the Jackson '017 patent is complex, and does not appear to be easily suitable for use in the manufacture of mass-produced lead-acid batteries. Nor does the device of the Jackson '017 patent disclose the specific indicator used to measure the output of the secondary cell. It appears likely, however, that the indicator contemplated by the Jackson patent, like the indicator of the Eby '942 patent, would be separate from the battery itself.
Also known in the art are automobile battery testers which measure the condition of a battery when put under load by, for example, the cranking of an associated automobile engine. One such tester is described in the previously identified '370 Holland patent. The Holland battery testing device compares the voltage of an unloaded battery with the voltage of the battery under load in order to determine whether a change in the battery voltage between the unloaded and loaded conditions is greater than a preset limit or alternatively whether the voltage is less than a preset value. However, prior art battery testers such as that taught by the Holland patent do not distinguish between an healthy battery merely in a state of low charge and a unhealthy battery near failure. Nor do such prior art testers isolate a failure between the battery and the automobile electrical system.
More recently, microprocessor technology has been incorporated into battery testers as exemplified by U.S. Pat. No. 4,725,784 to Peled et al. Although a microprocessor-based system for a battery tester provides an opportunity for complex testing, systems such as that in the '784 patent to Peled et al. do not distinguish between a healthy battery in a discharged state and an unhealthy battery nearing a failure state. The Peled et al. patent, for example, utilizes the power provided by a microprocessor to evaluate the state-of-charge of the battery in connection with a plurality of different sensor readings such as temperature and voltage recovery times. In connection with the execution of a test sequence for evaluating the state-of-charge of the battery, the Peled et al. device measures both the unloaded and loaded voltages across the terminals of the battery. However, these voltages are measured in isolation of other voltage measurements for the single purpose of determining whether the battery voltage is out of a range of values within which the battery voltage must stay in order for the test procedure to provide an accurate reading.
In the context of a lead-acid automobile starting, lighting and ignition (SLI) battery, it is highly desirable to include the monitoring device in the cover or side wall of the battery, so that the status of the battery can be readily determined without resort to separate testing apparatus. This permits the battery to be easily monitored when warehoused prior to installation in an automobile, and once installed, to be monitored by the automobile operator who is apt not to have a volt meter, hydrometer or other testing device.